1. Field of the Invention
The present invention relates to an image reading device in which image data is obtained by reading light that is one of transmitted through and reflected from frame images while a plurality of originals on which frame images are recorded are conveyed.
2. Description of the Related Art
There has been known in recent years an image forming technique in which a frame image recorded on an original such as a photographic film is photoelectrically read by a reading sensor such as a CCD, digital image data obtained by reading is subjected to image processing, such as enlargement, reduction and various corrections, and thereafter an image is formed on a recording material by a laser beam modulated on the basis of the digital image data which has been subjected to image processing.
In such an image reading technique, in which a frame image is digitally read by an image sensor such as a CCD, the frame image is read in a two-stage reading process. That is, in order to achieve accurate image reading, reading conditions corresponding to density of the frame image (for example, amount of light to be irradiated onto the frame image, the charge accumulation time of the CCD, and the like) are determined by conducting a preliminary reading of the frame image (known as pre-scanning) and then re-reading the frame image under the reading conditions determined from the pre-scanning (known as fine scanning).
In the above image reading system a halogen lamp, which is conventionally used for printing exposure and the like, is used as a light source. The halogen lamp, however, generates a large amount of heat when emitting light, which results in low efficiency of light emission. Accordingly, improvement of image reading speed has been limited for the image reading systems in which a halogen lamp is used.
A halogen lamp is most suitably used as a light source for direct printing on photographic printing paper by transmitting light through a negative film, as in the case of printing exposure. However, when a halogen lamp is employed in an image reading system which uses a CCD (for example, a linear CCD in which filters are attached, each of which corresponds to one of three primary colors) as described above, amounts of light of shorter wavelengths (blue (B) in particular) are low because of a low color temperature, which results in a poor signal-to-noise ratio of the image. Accordingly, improvement of image reading speed has been limited for image reading systems in which a halogen lamp is used.
Therefore, it has been proposed that LEDs be employed as a light source in an image reading system in which a CCD is used. Because an LED usually emits a specific color (blue: B, green: G, or red: R), a white light source can be formed by collectively disposing LEDs which emit each of these colors. Further, LEDs are suitable for use as light sources of image reading systems using CCDs, because of small amounts of heat generation and high color temperatures of LEDs.
Also, respective color filters can be attached to each line of a linear CCD such that each line detects density (amounts of light) for one of the colors.
In print processing of the above-described prior art, image readings of negative films of 135- (240-) size and the like are conducted one film at a time. Accordingly, the processing speed of printing is highly dependent on the speed at which each film is scanned. Particularly for a print process with film processing in which high-speed processing is required, further improvement of processing speed has been desired. However, further improvement of processing speed is not easily achieved, because scanning is conducted twice in order to achieve accurate image reading, as mentioned above.
Further, when image reading a large-size original, such as a Brownie-size film, it is necessary to provide LEDs having dimensions and light emission amounts corresponding to the size of the film. If the same device is to be used for reading images of both large-size originals and 135- (240-) size films, light emission capacity must be set to correspond to the large-size original, which may result in a problem of waste light being emitted from the light source (wasteful consumption of electric power) when images of 135- (240-) size film are read.